Electric power controls, particularly wall mounted dimmers are well known. U.S. Pat. Nos. 3,746,923 and 4,803,380, the disclosures of which are incorporated herein by reference, show typical light dimmer controls having a manually operable slide control. These devices conventionally have a controllable a-c current control device such as a triac mounted on a metal, typically aluminum, heat sink to dissipate heat generated by the triac during its operation. A control circuit which may be fixed to a suitable printed circuit board and a linearly adjustable potentiometer, which is connected to the slide control, is mounted atop, or adjacent, the triac. A rotary potentiometer may be used in place of the linearly adjustable potentiometer. The triac, potentiometer and control circuit are enclosed by a back cover. The external power conductors, such as insulated wires or stabs, which are connected to the interior triac, extend through the back cover for connecting the lighting load in series with an a-c source.
The control circuit is connected to the triac gate terminal to control the conduction phase of the triac, thus varying the power to the load. An on-off switch may also be provided in series with the triac to effect positive turn-off of power to the load when the slide or other control reaches an end position, or alternatively when a separate switch activator is operated.
The slide control is preferably mounted within an insulation adapter cover plate mounted on the surface of the heat sink plate opposite to the surface receiving the triac. A large number of other control schemes can also be used, including control by a microprocessor or coded radiation from a hand held radiation source.
Underwriters Laboratories, an independent approval agency, requires that the temperature rises measured at several strategic locations on the dimmer not exceed specified maximums. These temperature rises are measured over ambient temperature with the dimmer installed in a wall box in an insulated wall, with the dimmer carrying the maximum rated load. Further these temperature rises are the steady state temperature rise reached after the dimmer has been operating for an extended period of time. One requirement is that the back cover, which encloses all of the dimmer electronics, not exceed a 65.degree. C. temperature rise. This requires the selection of a particular type of triac (or other semiconductor switch) which has a sufficiently low power dissipation at the highest rated load current. Generally, a heat sink with fins and/or channels for flow of air past the heat sink have been needed to sufficiently cool the triac and wall box when load current is increased. Such devices require a large and relatively unsightly "stand-off" of the face plate of the dimmer from a wall. It has not been possible to increase the rating of an otherwise well designed low profile dimmer from 1500 watts to 2000 watts because the back cover temperature rise will exceed 65.degree. C. without a special heat sink. Thus, users of such low profile dimmers have to use plural 600 watt and/or 1500 watt units (without fins) to supply a 2000 watt load.
It would be very desirable to provide a 2000 watt dimmer which does not require exotic cooling techniques to achieve such power control without unduly increasing the temperature rise of the dimmer components such as the back cover and permitting a minimum stand-off of the dimmer face plate from the wall surface to which the dimmer is mounted.